Diamegnetism.

Diamagnetism is the property of an object
which causes it to create a magnetic field in
opposition of an externally applied magnetic
field, thus causing a repulsive effect.
Specifically, an external magnetic field alters the
orbital velocity of electrons around their nuclei,
thus changing the magnetic dipole moment in
the direction opposing the external field.
Diamagnets are materials with a magnetic
permeability less than μ0 (a relative
permeability less than 1).
Consequently, diamagnetism is a form of
magnetism that a substance exhibits only in the
presence of an externally applied magnetic field.
It is generally a quite weak effect in most
materials, although superconductors exhibit a
strong effect.